Styles was Christie's first published novel. It introduced Hercule Poirot, Inspector (later, Chief Inspector) Japp, and Arthur Hastings.[3] Poirot, a Belgian refugee of the Great War, is settling in England near the home of Emily Inglethorp, who helped him to his new life. His friend Hastings arrives as a guest at her home. When the woman is killed, Poirot uses his detective skills to solve the mystery. This is also the setting of Curtain, Poirot's last case.

The book includes maps of the house, the murder scene, and a drawing of a fragment of a will.

The story opens in England during World War I at Styles Court, an Essex country manor. Upon her husband's death, the wealthy widow, Emily Cavendish, inherited a life estate in Styles as well as the outright inheritance of the larger part of the late Mr. Cavendish's income. Mrs. Cavendish became Mrs. Inglethorp upon her recent marriage to a younger man, Alfred Inglethorp. Emily's two stepsons, John and Lawrence Cavendish, John's wife Mary and Cynthia Murdoch, also live at Styles. John Cavendish is the vestedremainderman of Styles; that is, the property will pass to him upon his stepmother's death, per his late father's will. Lawrence Cavendish would also come into a considerable sum of money. The income left to Mrs Inglethorp by her late husband would be distributed according to her will, which she changes at least once per year. If she has not changed her will since her marriage, her husband will inherit that income.[4] Cynthia does war-time work at the pharmacy in the nearby hospital.

The residents of Styles wake to find Emily Inglethorp dying of strychnine poisoning. Hastings, a house guest, enlists the help of his friend Hercule Poirot, who is staying in the nearby village, Styles St. Mary. Poirot pieces together events surrounding the murder. On the day she was killed, Emily Inglethorp was overheard arguing with someone, either her husband Alfred or her stepson John. Afterwards, she seemed quite distressed and, apparently, made a new will — which no one can find. She ate little at dinner and retired early to her room with her document case. The case was later forced open by someone and a document removed. Alfred Inglethorp left Styles earlier in the evening and stayed overnight in the nearby village, so was not present when the poisoning occurred. Nobody can explain how or when the strychnine was administered to Mrs. Inglethorp.

At first, Alfred is the prime suspect. He has the most to gain financially from his wife's death, and, since he is much younger than Emily was, the Cavendishes already suspect him as a fortune hunter. Evelyn Howard, Emily's companion, seems to hate him. His behaviour is suspicious; he openly purchased strychnine in the village before Emily was poisoned, and although he denies it, he refuses to provide an alibi. Inspector Japp is keen to arrest him, but Poirot intervenes by proving he could not have purchased the poison. Inspector Japp arrests John Cavendish. He inherits under the terms of her will, and there is evidence to suggest he had obtained poison.

Poirot clears Cavendish by proving it was Alfred Inglethorp who committed the crime, assisted by Evelyn Howard, who turns out to be his kissing cousin, not his enemy.[5] The guilty pair poisoned Emily by adding a precipitating agent, bromide (obtained from Mrs Inglethorp's sleeping powder), to her regular evening medicine, causing its normally innocuous strychnine constituents to sink to the bottom of the bottle where they were finally consumed in a single, lethal dose. Their plan had been for Alfred Inglethorp to incriminate himself with false evidence, which could then be refuted at his trial. Once acquitted, due to double jeopardy, he could not be tried for the crime a second time should any genuine evidence against him be subsequently discovered. When he realized that Alfred wanted to be arrested, Poirot made sure he was not arrested, until all the evidence against him was at hand.

Christie's mother, Clarissa ("Clara") Boehmer Miller (1854–1926), was a strong influence on her life and someone to whom Christie was extremely close, especially after the death of her father in 1901. It was while Christie was ill (circa 1908) that her mother suggested she write a story. The result was The House of Beauty, now a lost work which hesitantly started her writing career.[7] Christie later revised this story as The House of Dreams, and it was published in issue 74 of The Sovereign Magazine in January 1926 and, many years later, in 1997, in book form in While the Light Lasts and Other Stories.

Christie also dedicated her debut novel as Mary Westmacott, Giant's Bread (1930), to her mother who, by that time, had died.

The Times Literary Supplement (3 February 1921) gave the book an enthusiastic, if short, review, which stated: "The only fault this story has is that it is almost too ingenious." It went on to describe the basic set-up of the plot and concluded: "It is said to be the author's first book, and the result of a bet about the possibility of writing a detective story in which the reader would not be able to spot the criminal. Every reader must admit that the bet was won."[8]

Though this may be the first published book of Miss Agatha Christie, she betrays the cunning of an old hand ... You must wait for the last-but-one chapter in the book for the last link in the chain of evidence that enabled Mr. Poirot to unravel the whole complicated plot and lay the guilt where it really belonged. And you may safely make a wager with yourself that until you have heard M. Poirot's final word on the mysterious affair at Styles, you will be kept guessing at its solution and will most certainly never lay down this most entertaining book.

The novel's review in The Sunday Times of 20 February 1921, quoted the publisher's promotional blurb concerning Christie writing the book as the result of a bet that she would not be able to do so without the reader being able to guess the murderer, then said, "Personally we did not find the "spotting" so very difficult, but we are free to admit that the story is, especially for a first adventure in fiction, very well contrived, and that the solution of the mystery is the result of logical deduction. The story, moreover, has no lack of movement, and the several characters are well drawn."[10]

The contributor who wrote his column under the pseudonym of "A Man of Kent" in the 10 February 1921 issue of the Christian newspaper The British Weekly praised the novel but was overly generous in giving away the identity of the murderers. To wit,

It will rejoice the heart of all who truly relish detective stories, from Mr. McKenna[who?] downwards. I have heard that this is Miss Christie's first book, and that she wrote it in response to a challenge. If so, the feat was amazing, for the book is put together so deftly that I can remember no recent book of the kind, which approaches it in merit. It is well written, well proportioned, and full of surprises. When does the reader first suspect the murderer? For my part, I made up my mind from the beginning that the middle-aged husband of the old lady was in every way qualified to murder her, and I refused to surrender this conviction when suspicion of him is scattered for a moment. But I was not in the least degree prepared to find that his accomplice was the woman who pretended to be a friend. I ought to say, however, that an expert in detective stories with whom I discussed it, said he was convinced from the beginning that the true culprit was the woman whom the victim in her lifetime believed to be her staunchest friend. I hope I have not revealed too much of the plot. Lovers of good detective stories will, without exception, rejoice in this book.[11]

The Bodley Head quoted excerpts from this review in future books by Christie but, understandably, did not use those passages which gave away the identity of the culprits.

"Introducing Hercule Poirot, the brilliant – and eccentric – detective who, at a friend's request, steps out of retirement – and into the shadows of a classic mystery on the outskirts of Essex. The victim is the wealthy mistress of Styles Court, found in her locked bedroom with the name of her late husband on her dying lips. Poirot has a few questions for her fortune-hunting new spouse, her aimless stepsons, her private doctor, and her hired companion. The answers are positively poisonous. Who's responsible, and why, can only be revealed by the master detective himself." (Book jacket, Berkley Book edition April 1984)

In his book, A Talent to Deceive – An Appreciation of Agatha Christie, writer/critic Robert Barnard wrote:

Christie's debut novel, from which she made £25 and John Lane made goodness knows how much. The Big House in wartime, with privations, war work and rumours of spies. Her hand was over-liberal with clues and red herrings, but it was a highly cunning hand, even at this stage[12]

In general The Mysterious Affair at Styles is a considerable achievement for a first-off author. The country-house-party murder is a stereotype in the detective-story genre, which Christie makes no great use of. Not her sort of occasion, at least later in life, and perhaps not really her class. The family party is much more in her line, and this is what we have here. This is one of the few Christies anchored in time and space: we are in Essex, during the First World War. The family is kept together under one roof by the exigencies of war and of a matriarch demanding rather than tyrannical – not one of her later splendid monsters, but a sympathetic and lightly shaded characterisation. If the lifestyle of the family still seems to us lavish, even wasteful, nevertheless we have the half sense that we are witnessing the beginning of the end of the Edwardian summer, that the era of country-house living has entered its final phase. Christie takes advantage of this end-of-an-era feeling in several ways: while she uses the full range of servants and their testimony, a sense of decline, of break-up is evident; feudal attitudes exist, but they crack easily. The marriage of the matriarch with a mysterious nobody is the central out-of-joint event in an intricate web of subtle changes. The family is lightly but effectively characterised, and on the outskirts of the story are the villagers, the small businessmen, and the surrounding farmers – the nucleus of Mayhem Parva. It is, too, a very clever story, with clues and red herrings falling thick and fast. We are entering the age when plans of the house were an indispensable aid to the aspirant solver of detective stories, and when cleverness was more important than suspense. But here we come to a problem that Agatha Christie has not yet solved, for cleverness over the long length easily becomes exhausting, and too many clues tend to cancel each other out, as far as reader interest is concerned. These were problems which Conan Doyle never satisfactorily overcame, but which Christie would.".[13]

In the "Binge!" article of Entertainment Weekly Issue #1343-44 (26 December 2014–3 January 2015), the writers picked The Mysterious Affair at Styles as an "EW favorite" on the list of the "Nine Great Christie Novels".[14]

The story is told in the first person by Hastings, and features many of the elements that have become icons of the Golden Age of Detective Fiction, largely due to Christie's influence. It is set in a large, isolated country manor. There are a half-dozen suspects, most of whom are hiding facts about themselves. The plot includes a number of red herrings and surprise twists.[citation needed]

The Mysterious Affair at Styles was adapted as a 103-minute drama and transmitted on ITV in the UK on Sunday, 16 September 1990, as a special episode in their series Agatha Christie's Poirot to celebrate the centenary of the author's birth.[15]

The adaptation was generally faithful to the novel, although Dr Bauerstein and some minor characters were left out. Adaptor: Clive ExtonDirector:Ross Devenish

The novel was adapted as a five-part serial for BBC Radio 4 in 2005. John Moffatt reprised his role of Poirot. The serial was broadcast weekly from Monday, 5 September to Monday, 3 October, from 11.30 am to 12.00 noon. All five episodes were recorded on Monday, 4 April 2005, at Bush House. This version retained the first-person narration by the character of Hastings.[citation needed]

On 14 February 2012, Great Lakes Theater debuted a 65-minute stage adaptation as part of their educational programming. Adapted by David Hansen, this production is performed by a cast of five (3 men, 2 women) with most performers playing more than one role. Cleveland.com called this stage adaptation, "charming, funny and smart ... a quick-paced gem, and a thrill for any detective aficionado."[17]

1931, John Lane (The Bodley Head), February 1931 (As part of the Agatha Christie Omnibus along with The Murder on the Links and Poirot Investigates), Hardcover, Priced at seven shillings and sixpence; a cheaper edition at five shillings was published in October 1932

96 Kindle editions from December 2001 to November 2013 (ISBN B008BIGEHG).[18]

The novel received its first true publication as an eighteen-part serialisation in The Times newspaper's Colonial Edition (aka The Weekly Times) from 27 February (Issue 2252) to 26 June 1920 (Issue 2269).[19] This version of the novel mirrored the published version with no textual differences and included the maps and illustrations of handwriting examples used in the novel. At the end of the serialisation an advertisement appeared in the newspaper, which announced, "This is a brilliant mystery novel, which has had the unique distinction for a first novel of being serialised in The Times Weekly Edition. Mr. John Lane is now preparing a large edition in volume form, which will be ready immediately." Although another line of the advertisement stated that the book would be ready in August, it was first published by John Lane in the United States in October 1920 and was not published in the UK by The Bodley Head until the following year.

The Mysterious Affair at Styles later made publishing history by being one of the first ten books to be published by Penguin Books when they were launched on 30 July 1935. The book was Penguin Number 6.[20]

The blurb on the inside flap of the dustwrapper of the first edition reads:

This novel was originally written as the result of a bet, that the author, who had previously never written a book, could not compose a detective novel in which the reader would not be able to "spot" the murderer, although having access to the same clues as the detective. The author has certainly won her bet, and in addition to a most ingenious plot of the best detective type she has introduced a new type of detective in the shape of a Belgian. This novel has had the unique distinction for a first book of being accepted by the Times as a serial for its weekly edition.